Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical layout of a conventional fuel cell 10 is shown in FIG. 1 which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane 11 is sandwiched between an anode 12 and a cathode 13. The polymer membrane allows protons to traverse the membrane, but blocks the passage of electrons. Typically, the anode 12 and the cathode 13 are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode 12 and cathode 13 are often bonded directly to the respective adjacent surfaces of the membrane 11. This combination is commonly referred to as the membrane-electrode assembly, or MEA.
Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate 14 and a cathode fluid flow field plate 15. The fluid flow field plates 14, 15 are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode 12 or cathode electrode 13. At the same time, the fluid flow field plates must enable the delivery and/or exhaust of fluid fuel, oxidant and/or reaction products (and/or other diluent gases not taking part in the reaction) to or from the porous electrodes. This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels 16 in the surface presented to the porous electrodes.
FIG. 2 shows a plan view of a typical fluid flow channel 16 arranged as a serpentine structure 20 in a face of the anode 14 (or cathode) having an inlet manifold 21 and an outlet manifold 22. Many different configurations of fluid flow channel may be used.
In a typical application, in the anode fluid flow field plate 14, hydrogen gas is delivered into the serpentine channel 20 from the inlet manifold 21. In the cathode fluid flow field plate 15, oxidant (eg. oxygen gas) is delivered into the serpentine channel 20 from the inlet manifold.
Prior to the start up of a fuel cell 10 after first assembly, commissioning, repair, prolonged periods of inactivity or stand-by there can be an accumulation of air in the fuel flow channels and fuel delivery conduits, ie. generally within the fuel delivery path of the fuel cell. There is therefore a need to remove this air, or more particularly to remove the oxygen in the air, from the anode fuel delivery path before the introduction of any hydrogen fuel or a hydrogen rich gas mixture to the anode 12 and membrane 11.
This removal of oxygen prior to delivery of fuel is important to prevent undesirable uncontrolled catalytic combustion occurring at the surface of the anode 14 resulting in localized heating, dehydration and possible puncture of the proton exchange membrane 11.
In the prior art, it is common practice to purge the anode channels 16 and other portions of the fuel delivery conduits by passage of an inert gas, such as nitrogen, for a period of time prior to introduction of hydrogen fuel.
This process necessitates a local supply of nitrogen, generally contained in a pressure cylinder, and its periodic replacement. It is desirable to eliminate this requirement and thereby simplify the operational and service needs of the system. This is particularly important when the fuel cell is already installed in the field, eg. as part of a power system in a vehicle where accessibility of a purge gas, and indeed accessibility to the fuel cell, may be limited.